Many industrial and ground waters are contaminated with oxyanions and other anions. There are a number of such contaminants that may be found in water including chromate, bichromate, dichromate, hexavalent chromium, selenite, selenate, arsenate, arsenite, perchlorate, iodate, bromate, vanadate, antimonite, antimonate, molybdate, phosphate, borate, fluoride, iodide, tungstanate, molybdate, bromide, chloride, and sulfate. Many of these oxyanions are present in the fossil fuels and hence migrate to the wastewater from the industries that refine these fossil fuels for commercial use. Many of these oxyanions are toxic to the health and environment. The release of many of these oxyanions to the environment is regulated by federal and local regulatory agencies. For example, the discharge limit for arsenic is 10 ppb for direct discharge. In certain ecologically sensitive areas it can be as low as 5 ppb total arsenic. Vanadate is also regulated for direct discharge. Generally arsenic is present as arsenite and arsenate depending on the pH and the oxidative properties of water. There are a number of technologies available for arsenate removal by first oxidizing arsenite to arsenate followed by co-precipitation/adsorption with iron salts. This method is effective but does generate a large volume of sludge. The disposal of the iron based sludge in a landfill is a common practice. The anaerobic conditions in a landfill will transform arsenate back to arsenite which would be mobilized to leachates. Chemical/precipitation also requires large foot print, high chemical and high sludge disposal cost making the overall treatment quite expensive. Ion exchange resins will also incur high costs due to frequent regeneration of the resins due to presence of sulfate commonly found in water/wastewater.
Chromium is a common heavy metal contaminant of water supplies, largely arising from the chrome plating, textile, leather tanning, and wood preservation treatment industries. Hexavalent chromium in groundwater originates due to past industrial activities such as chrome plating, cooling tower operation, and leather tanning. While trivalent chromium is an essential trace element for humans and plays an important role in the sugar regulation and fat metabolism, hexavalent chromium is very toxic to flora and fauna.
Hexavalent chromium is known for its negative health impact and is considered carcinogenic and mutagenic even at very low concentrations. It also causes allergic and asthmatic reactions and it is 1000 times more toxic than trivalent chromium. Exposure to hexavalent chromium causes diarrhea, stomach and intestinal bleeding, cramps, paralysis and liver and kidney damage. The treatment limits for hexavalent chromium are becoming very stringent and some states have lowered the acceptable treatment level to 10 ppb hexavalent chromium in treated water. Regulatory agencies demand that hexavalent chromium contamination is effectively removed to make water safer.
One of the current technologies to remove hexavalent chromium from water is to use polymeric anion exchange resins such as strong base anion (SBA) exchange resins. However, these regenerable ion exchange resins are non-selective and hence remove other competing anions such as sulfate requiring frequent regeneration of the resins. This frequent regeneration produces large quantities of spent regenerant containing high concentration of hexavalent chromium. The regenerant needs further treatment prior to appropriate disposal.
Another technology used is the use of weak base anion (WBA) exchange resins which is gaining acceptance due to an overall cost reduction compared to SBA exchange resins. However, one of the limitations with WBA resins is the requirement to lower the pH of the water (influent) to about 5 prior to treatment for optimum performance of these resins. The treated water (effluent) may also require another pH adjustment to raise the pH back to neutral. These pH adjustments are expensive. It is desirable that a product be developed which works in the normal pH range (neutral) of the typical chromium contaminated groundwater.
Yet another method for removal of hexavalent chromium from water is reduction-coagulation-filtration (RCF). In this process, hexavalent chromium is reduced to trivalent state by a reductant such as ferrous ions following precipitation and coagulation. The solids are generally separated by filtration. However, the current process is expensive and labor intensive. Further, it also generates large volume of sludge for disposal.
Another anion that may be present in some industrial waste waters is selenium. Selenium is an essential nutrient but it is required in extremely small quantities. At higher concentration it is toxic and poses risk to health and the environment for example skeletal deformities are observed in fish exposed to higher selenium concentration. Therefore, release of selenium to the environment is regulated by federal and local regulatory agencies. The selenium discharge requirements are becoming stringent and are often in the range of 5-12 ppb. Selenium is often found in fossil fuels such as coal and crude oil. Many industrial wastewaters such as petroleum refinery wastewater, mining leachate, coal mining and agricultural drainage water contain selenium. In petroleum refinery wastewater most of the selenium is present as selenite with small quantity of selenate. The total selenium present is normally in the range of a few hundred ppb as total selenium. These waters also contain interfering anions such as sulfate and chloride making existing technologies such as ion exchange resins cost prohibitive for commercial operation. Layered metal hydroxy salts have shown that total selenium can be removed to very low concentrations which could enable the industries to meet stringent discharge limit for total selenium.
Selenite can be removed by chemical/precipitation treatment such as by use of iron salts however, large quantities of these iron salts are needed resulting in very large quantities of sludge which needs to be disposed. Chemical/precipitation also requires large foot print, high chemical cost and large cost for sludge disposal making the treatment very costly. Ion exchange resins will also incur high costs due to frequent regeneration of the resins due to presence of sulfate. Use of copper salts has also been described however, it is not commercially viable since excess copper remains in the effluent making the treated water unacceptable for direct discharge.
Accordingly, it is desirable to provide materials, methods and apparatuses to remove hexavalent chromium, selenium, arsenic and other contaminants from contaminated water to meet stringent water quality requirements. Further, it is desirable to provide cost-effective materials, methods and apparatus that does not require pH adjustment for removal of hexavalent chromium from contaminated water. Moreover, it is desirable to provide materials, methods and apparatuses for preferential removal of hexavalent chromium from contaminated water containing competing anions. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the background of the subject matter.